Murchison Widefield Array Report

Site update

The deployment of the long baseline tiles required for the extended Murchison Widefield Array (MWA) Phase 2 configuration saw major progress this quarter. All of the cabling, mesh and dipoles required for the 56 new tiles were successfully positioned and assembled by Geraldton-based company Gco Electrical (GCo), with supervision from members of the MWA operations team. GCo led the original site infrastructure construction program for MWA Phase 1 in 2012 but have not previously been involved with the assembly and installation of telescope system components. The experience gleaned by GCo in through their involvement in the Phase 2 deployment will be critical to the success of MWA’s planned transition to an industry led routine maintenance model over coming months.

One of the MWA’s 56 new long baseline antenna tiles being assembled in the field.

Data compression

Compression of MWA data to make more efficient use of the storage resources available at the Pawsey Centre has been an important objective for some time. The addition of a dedicated MWA Data Manager from mid-2016 has enabled significant progress toward this objective in recent times.

A lossless RICE-compression technique, extensively tested with MWA science teams through the early part of 2017, has been implemented on the MWA GPU-cluster (correlator) on the MRO such that all new data taken by the MWA is now compressed before archiving. The compression ratio achieved ranges from 2:1 to 3:1 depending on the data (instrument observing mode). Offline compression of the roughly 15 PB of MWA data already archived in the Pawsey Centre will occur as an automated background process over an extended period of time.

MWA Membership

The MWA institutional membership has grown to 21 partners with the Shanghai Astronomical Observatory (SAO) committing to the Collaboration. SAO’s membership has been endorsed by the MWA Board and will be formalised as part of the transition to a Phase 2 Collaboration Agreement that is currently in preparation.

Engineering Development Array (EDA) Status and results

The Engineering Development Array (EDA) was the first SKA-low sized station prototype system, brought online in mid-2016. It consists of 256 modified MWA dipoles and analogue beamformers, working as a single phased-array station. The EDA was connected to an MWA receiver to directly measure its sensitivity, as well as using the station in an independent “drift scan” mode to measure sensitivity and receiver temperature.

A view of the EDA looking north. Dipole antennas are connected to beamformers (white boxes). Signals from the beamformers are combined to form a station beam in an equipment hut ( outside of photo on left).

The EDA’s sensitivity was measured from 60 to 240 MHz while connected to the MWA, and the results are in very good agreement with expectations from laboratory measurements of the receiver temperature and simulations of the array. A full description of this process, and the results can be found in Wayth et al, 2017 PASA.

In the coming few months, the EDA will be re-purposed to provide a reliable sky signal to the prototype SKA-low Tile Processing Modules (TPMs) which are currently installed at the Murchison Radio-astronomy Observatory (MRO) as part of MWA’s ongoing support for the AAVS1 system (see AADC report).

MWA Science

The MWA continues to be scientifically productive and there are now over 90 collaboration-led publications since the commencement of operations in mid-2013. The publications cover a diverse range of science areas, from solar science through to the Epoch of Reionization.

The following are some science highlights from the recent quarter.

Wavelet-based Characterization of Small-scale Solar Emission Features at Low Radio Frequencies”Suresh, Sharma, Oberoi et al.

Suresh, Sharma, Oberoi et al. 2017 ApJ.

The solar emission at low radio frequencies observed by the MWA often shows the presence of numerous short-lived features. These features often last only for about a second or so, span a few MHz and range in strength from a few percent to many 100s times the background solar flux. These features occur at rates of many thousands per hour, so they are too numerous to identify and characterise manually. Using a type of machine learning algorithms, a team of scientists (Dr Divya Oberoi, NCRA-TIFR, India and collaborators including student Ashkay Suresh) have developed an algorithm to automate the detection and characterise these emission features in MWA data. In fact, the very existence of the weaker class of these emissions was first established using data from the MWA prototype by the same team in their earlier work in 2011. It is important to characterise these weak emission features as they might hold the clues to unravelling the long-standing coronal heating problem. Very interestingly, the emissions studied here are found to have the right characteristics to contribute some of the missing heat flux to the corona.

Figure from Suresh et al., 2017 showing raw (left) and calibrated dynamic spectra (right) from the MWA.

Low frequency continuum observations of pulsars with the Murchison Widefield Array” Murphy et al., PASA 2017.

Pulsars are typically associated with short-timescale (millisecond) bursts of radio waves that need special high time resolution capabilities to observe. At low radio frequencies, however, many pulsars appear as normal-looking radio sources with steady radio emission. The very compact size of pulsars means that they do scintillate, however, which is a phenomenon that can be used to study the tenuous interstellar medium. This paper presents the continuum radio properties of 60 pulsars observed as part of the large MWA Transients Survey (MWATS).

154 MHz images of Pulsar J0828-3417 in its off (left) and on (right) states, found in two images from the MWA Transients survey (MWATS). The two images are separated by 6 minutes.

Report provided by Randall Wayth